fix(cli): fail closed when messages get targets a nonexistent channel

[sanjay3290]

Fixes #6389.

Problem

buzz messages get --channel <uuid> prints [] and exits 0 when the channel does not exist or the caller is not a member. That is indistinguishable from a channel with no messages, so scripts cannot tell "no messages yet" from "wrong UUID".

Change

cmd_get_messages probes for the channel's kind:39000 discovery event by #d before querying messages. If the probe returns no event, it returns CliError::NotFound, which exits 1.

This also covers the unauthorized case from the issue: kind:39000 events are stored channel-scoped, so a non-member's probe comes back empty and the command fails closed rather than reporting an empty history.

The probe filter matches the one cmd_get_channel already uses in crates/buzz-cli/src/commands/channels.rs.

Before / after

$ buzz messages get --channel 00000000-0000-4000-8000-000000000000 --limit 1
[]
$ echo $?
0

$ buzz messages get --channel 00000000-0000-4000-8000-000000000000 --limit 1
error: channel '00000000-0000-4000-8000-000000000000' not found
$ echo $?
1

Tests

Two unit tests in crates/buzz-cli/src/commands/messages.rs, each against a local stub relay:

• cmd_get_messages_fails_when_channel_not_found — probe returns [], expect CliError::NotFound
• cmd_get_messages_succeeds_when_channel_exists — probe returns a kind:39000 event, expect Ok and the messages printed

The first test fails on main with expected NotFound, got Ok(()).

Verification

Run on this branch, rebased on 24ec6a4:

cargo clippy --workspace --all-targets -- -D warnings   # exit 0
cargo fmt --all -- --check                              # exit 0
cargo test -p buzz-cli --lib                            # 365 passed, 0 failed

Notes

• Costs one extra relay query per messages get. That seemed the right trade for a correct exit code, but I am happy to gate it behind a flag if you would rather not pay it on the hot path.
• Scoped to messages get only. messages thread, channels members, and canvas get have the same silent-empty behaviour and are left alone here.